This invention relates to a process for removing sulfur from a hydrocarbon conversion catalyst which has become contaminated with sulfur during use in a hydrocarbon conversion system, particularly to the in situ in removal of sulfur from a catalyst containing platinum and alumina which has become deactivated and sulfur contaminated.
Catalysts used in such hydrocarbon conversion processes as reforming, cracking, isomerization, alkylation, etc., normally become deactivated after a period of catalytic use. It is known to regenerate such catalysts by treating them with an oxygen-containing gas to burn off deactivating substances such as coke. Regeneration of hydrocarbon conversion catalysts with oxygen has often been performed in situ, i.e., by leaving a bed of catalyst in the reactor in which it had been used for catalytic conversion and passing an oxygen-containing regeneration gas into the reactor and through the catalyst bed to burn the deactivating substances off the catalyst.
Many hydrocarbon conversion systems, particularly naphtha hydroreforming units, employ a process scheme which passes the feed in series flow through two or more separate reactors. Each of the reactors contains at least one bed of a catalyst, and each reactor is connected to one or more other vessels by large-sized conduits through which the feed is passed. When the hydrocarbon conversion reaction, or reactions, being carried out in a particular system are endothermic, as in reforming, a heater, such as a heat exchanger or furnace, is normally connected into each of the large feed conduits upstream of each reactor to preheat the feed.
During in situ regeneration of a catalyst in plural-reactor-vessel conversion units, an oxygen-containing gas is conventionally passed in series flow through all the reactor vessels. The large feed conduits running between the reactors are used to pass the oxygen-containing gas from one reactor vessel to the next.
Hydrocarbons used as charge stocks for hydrocarbon conversion systems often contain sulfur compounds. During the operation of a conversion unit, sulfur in the feed reacts with iron in the linings of reactor, furnaces and other vessels, forming iron-sulfide scale. To some extent, sulfur may also be deposited directly in catalyst beds during hydrocarbon processing. In catalytic reforming units, sulfur contamination is usually a problem primarily in the furnaces and heat exchangers employed to heat the feed before it is charged to the reactors.
When a catalyst is regenerated in situ in a unit which has been contaminated with sulfur, oxygen in the regenerating gas reacts with sulfide scale to form sulfur dioxide. Some hydrocarbon conversion catalysts, e.g., those containing platinum and alumina, catalyze the reaction of sulfur dioxide and oxygen in the regeneration gas to form sulfur trioxide. Sulfur trioxide may then react with alumina in the catalyst to form sulfate, partially displacing catalytic metals such as platinum and rhenium. This impedes redistribution of catalytic metals on the alumina. Since good redistribution of catalytic metals is an important part of proper catalyst regeneration, the presence of sulfate in the catalyst during oxidative, high-temperature regeneration is undesirable.
Chemical cleaning of the whole conversion system effectively removes sulfur from all the vessels, and thus prevents substantial sulfate contamination of catalyst beds, but is costly and time-consuming. The present invention provides an economical and easily performed process for removing contaminant sulfate from a catalyst bed when the reactor containing the bed is connected to other vessels such as furnaces and reactors by large feed conduits. The present process prevents sulfur contamination of other reactors, catalyst beds, and heaters such as furnaces in a conversion system.
It has previously been suggested to isolate various heavily sulfur-contaminated elements of a hydrocarbon conversion unit, such as heaters, before contacting a deactivated catalyst with an oxygen-containing regeneration gas and to purge iron sulfide from the isolated heaters with high-temperature steam and/or an oxygen-containing gas. The freed particles of sulfide scale and/or sulfur dioxide-containing gas are then removed from the system. This type of vessel cleaning requires that several expensive valves or similar apparatus be installed directly into large feed conduits. Such apparatus is expensive and is unnecessary for normal operation of the conversion system and is used solely in the relatively infrequent regeneration of a catalyst. This procedure also requires unusually high-temperature conditions. The above-mentioned and other procedures for eliminating sulfide scale from heaters and other sulfur-contaminatable vessels in hydrocarbon conversion units are deficient in failing to provide an effective method for removing sulfur, as the sulfate, from a catalyst bed during in situ catalyst regeneration, hindering proper redistribution of catalytic metals on an alumina catalyst base.